Thank you for your replies!
This update runs for 45-minutes and then reports it has failed. This happens in Windows Update and manually installing it after downloading from MS Catalog.
KB4565912 is installed and other updates are installing so this problem specific to KB4565511.
sfc /scannow > Ran this several times as Admin but it did not help.
dism /online /cleanup-image /restorehealth > Ran this as Admin and, after an hour of no progress, not even a progress bar, I cancelled it. Any recommendations on getting it to run?
I have also tried clearing the SoftwareDistribution folder.
It is a production Exchange server so rebuilding it would be far more costly than paying Microsoft $500 to fix it but I don't want to do that until I am sure I have tried everything possible.
It has been said that the CBS.log could hold a clue to fixing this problem but I can't make much sense of it.
Any other suggestions?